Differential microwave microfluid sensor based on complementary open-loop resonator structure
Technical Field
The invention belongs to the technical field of microwaves, relates to a microstrip line excitation sensor, and particularly relates to a differential microwave microfluidic sensor for measuring the complex dielectric constant of an electrolyte solution based on a complementary split-ring resonator (CSRR).
Background
Complex dielectric constant (. epsilon.) of electrolyte solutionr=ε′r(1-jtanδe),ε′rDenotes the dielectric constant, tan. deltaeRepresenting loss values) is crucial for the fields of bio-electromagnetism and microwave chemistry. The microwave interacts with the electrolyte, wherein the absorption rate of the microwave energy is closely related to the complex dielectric constant of the electrolyte. In industry, the complex dielectric constants of various electrolyte materials are known, and the absorption and reflection conditions of the materials to microwaves can be further known, which is of great significance for improving the utilization rate of microwave energy and the like.
With the rapid development of microwave technology in various industries (such as military, medicine, food, chemical and meteorology fields), various types of radio frequency microwave devices are gradually developed and applied, wherein the research of microwave sensors for measuring the complex dielectric constant of electrolyte materials has been receiving wide attention of scholars. There are many methods for measuring the complex dielectric constant, and the methods are mainly classified into a resonance method and a non-resonance method. The most typical method in the resonance method is a resonant cavity method, and the measurement method has almost no interference of external factors on measurement, so that the method is the most accurate method for measuring the complex dielectric constant of the electrolyte material. The design idea of the resonant cavity method is to place a sample to be measured with a fixed size into a set position in the resonant cavity, and then reversely deduce the complex dielectric constant of the sample to be measured according to the change of S parameters of the resonant cavity and the difference of quality factor Q values. In existing miniaturized microwave sensors based on the resonance principle, the complex permittivity sensing characterization of the electrolyte material has the same properties, i.e. they all reduce the resonance frequency and the quality factor. The variation of the resonant frequency determines the transverse sensitivity of the sensor, and the larger the variation of the resonant frequency is, the higher the transverse sensitivity is; the magnitude of the quality factor and the amount of variation determine the longitudinal sensitivity of the sensor, and the higher the quality factor, the greater the amount of variation, the higher the longitudinal sensitivity. However, the main evaluation indexes of the sensor performance are whether the sensor has high enough sensitivity and low enough measurement error, the lateral sensitivity of the existing microwave microfluidic sensor based on the CSRR resonator and used for measuring the complex dielectric constant of the dielectric solution is lower than 7.0(MHz), the sensor is easily influenced by certain surrounding environmental factors, and the non-negligible measurement error is not considered, so that the measurement is not accurate. In recent years, some scholars in the microwave field at home and abroad design microwave microfluidic sensors capable of effectively improving transverse sensitivity. For example, Amir Ebrahimi has published a periodical paper "High-Sensitivity mechanical-induced Sensor for Microfluidic Dielectric chromatography" based on CSRR, and the transverse Sensitivity of the periodical paper reaches 5.0MHz, which is the highest transverse Sensitivity in the current Sensor based on CSRR; however, the lateral sensitivity is increased while the cross sensitivity of the surrounding environment is necessarily increased, which causes a non-negligible measurement error, and thus the measurement accuracy of the sensor is low. To address the interference of environmental factors, some scholars have introduced differential structures in the sensor design to address this issue. For example, Paris Velez has published a journal article "microwave microfluidic sensor based on a microstrip site/combiner configuration and differential characterization of Semiconductors (SRRs)" based on differential structure, which effectively eliminates the influence of surrounding environmental factors, and has high measurement accuracy but lateral sensitivity lower than 5.0 MHz. To above problem, the design of this application structure mainly improves lateral sensitivity (>7.0), improves measurement accuracy and miniaturized sensor improvement practicality simultaneously.
Disclosure of Invention
The invention aims to provide a differential microwave microfluidic sensor which is simple in structure, high in sensitivity, high in Q value and wide in measurement range, and mainly aims at overcoming the defects of the prior art. The sensor is designed based on a conventional complementary open-loop resonator and transmission line structure.
The invention is realized according to the following technical scheme:
a differential microwave microfluid sensor is a two-port device and comprises a microstrip line structure, a dielectric layer, a metal sheet and two grooved metal CSRR structures;
the microstrip line is arranged on the upper surface of the dielectric layer; the microstrip line structure comprises a microstrip line I, a microstrip line II, a microstrip line III, a microstrip line IV and a microstrip line V, wherein the microstrip line IV and the microstrip line V are arranged in an axial symmetry manner, and the microstrip line II and the microstrip line III are arranged in an axial symmetry manner; the microstrip line II is formed by integrally forming a first microstrip line and a second microstrip line which are perpendicular to each other, the microstrip line III is formed by integrally forming a third microstrip line and a fourth microstrip line which are perpendicular to each other, one end of the microstrip line I is connected with one end of the first microstrip line through a 50-ohm resistor, and the other end of the microstrip line I is connected with one end of the third microstrip line through a 50-ohm resistor. The microstrip line I, the first microstrip line and the third microstrip line are located on the same straight line. The microstrip line IV is formed by integrally molding a fifth microstrip line and a sixth microstrip line, the fifth microstrip line is a rounded broken line (namely a 90-degree smooth swept elbow for eliminating parasitic discontinuous capacitance between the microstrip line II and the microstrip line IV), one end of the fifth microstrip line is connected with the middle point of the sixth microstrip line, and the other end of the fifth microstrip line is used as an input/output port; the microstrip line V is formed by integrally molding a seventh microstrip line and an eighth microstrip line, the seventh microstrip line is a rounded broken line (namely a 90-degree smooth sweep elbow for eliminating parasitic discontinuous capacitance between the microstrip line III and the microstrip line V), one end of the seventh microstrip line is connected with the middle point of the eighth microstrip line, and the other end of the seventh microstrip line is used as an input/output port. The sixth microstrip line and the second microstrip line are arranged in parallel, and a gap is reserved; the eighth microstrip line and the fourth microstrip line are arranged in parallel, and a gap is left. The gap distance is preferably 7 mm. The lengths of the sixth microstrip line, the second microstrip line, the eighth microstrip line and the fourth microstrip line are preferably 12 mm.
The input and output port is used for connecting an SMA connector, and the SMA connector is communicated with the vector network analyzer.
Furthermore, a through hole penetrating through the microstrip line I, the dielectric layer and the metal sheet is formed in the center of the microstrip line I, and the peripheral wall of the through hole is metalized so that the microstrip line I is communicated with the metal sheet at the bottom layer;
furthermore, the lengths of the fifth microstrip line and the seventh microstrip line are integral multiples of 1/4 wavelength, and are set to be 35mm, and the widths are set to be 2.73 mm;
furthermore, the total length of the microstrip line I and a 50-ohm resistor is 30mm, and the width of the microstrip line I and the 50-ohm resistor is set to be 2.73 mm;
further, the dielectric layer is a square PCB;
the metal sheet is the same as the dielectric layer in shape, is arranged on the lower surface of the dielectric layer, is etched with two slotted metal CSRR structures, and has the same opening direction. The groove-carving metal CSRR structure is coupled with the microstrip line II and the microstrip line III.
Each grooved metal CSRR structure is a square groove ring, the groove ring is provided with an opening, the electric field intensity of a groove area which is connected with two right angles opposite to the opening of the groove ring is the largest, and a microfluidic chip is placed in the groove area and used for measuring the complex dielectric constant of an electrolyte solution;
the microfluidic chip is manufactured by processing low-cost Polydimethylsiloxane (PDMS), is a U-shaped chip, and is internally provided with a microfluidic channel for storing electrolyte solution; the microflow channel is positioned above the groove; one of the microfluidic channels serves as a measurement and the other serves as a reference.
The center of the first grooved metal CSRR structure is consistent with the center of the gap between the microstrip line II and the microstrip line IV on the plane position, and the closest distance between the outer side of the second microstrip line and the outer side of the first grooved metal CSRR structure in the x-axis direction is p2Preferably 0.5 mm. The nearest distance between the two ends of the second microstrip line and the outer side of the first grooved metal CSRR structure in the y-axis direction is p4Preferably 3.5 mm.
Furthermore, the size of the groove ring of the groove-carved metal CSRR structure is set to be 19mm multiplied by 10mm, the groove width is 1mm, the width of the opening of the groove ring is 0.5mm, and the reasonable size of the groove ring enables an electric field to be well bound on the periphery of the groove ring;
the lateral sensitivity of the sensor determines the resolution of the dielectric constant of the electrolyte solution; the quality factor Q value determines the resolution of the electrolyte solution loss; the measuring range and miniaturization determine the practicality of the sensor.
Compared with the prior art, the invention has the following prominent substantive characteristics and remarkable technical progress:
compared with the existing microwave microfluidic sensor based on the CSRR resonator, the invention firstly adopts the T-shaped microstrip line to couple the CSRR, so that the coupling strength between the microstrip line and the CSRR is effectively improved, and the electric field intensity of a groove area which is tightly bound at the edge of a CSRR groove ring and is connected between two right angles opposite to the opening of the groove ring reaches the maximum; and secondly, a design mode of a differential structure is adopted, so that the interference of surrounding environment factors is effectively eliminated. Based on the design of the T-shaped microstrip line and the differential structure, the sensor overcomes the defects of low transverse sensitivity and large measurement error of the existing sensor, has extremely high transverse sensitivity and Q value, and ensures the measurement accuracy. The restriction of the notched metal CSRR structure of the sensor to a strong field is strong, so that the transverse sensitivity is high, and meanwhile, the coupling between the microstrip line II and the microstrip line III and the notched metal CSRR structure improves the impedance matching during CSRR resonance, thereby improving the quality factor. In addition, the invention adopts a differential structure form to perform differential measurement on the complex dielectric constant, and eliminates the influence of environmental factors by adopting a relative measurement mode.
Drawings
FIG. 1 is a schematic diagram of the structure and parameter labeling diagram of the present invention: wherein (a) a schematic top sensor layer, (b) a schematic bottom sensor layer, (c) a schematic plan sensor layer;
FIG. 2 is a schematic diagram of the electric field intensity distribution of the present invention;
FIG. 3 is a schematic diagram of the structure and parameter labeling of the microfluidic chip according to the present invention;
FIG. 4 is a schematic diagram of a three-dimensional hierarchical layout structure of the present invention;
FIG. 5 is a schematic of the sensor S parameters of the present invention: the system comprises a first sensing unit (left), a second sensing unit (right), a micro-fluidic chip and a micro-fluidic chip, wherein the first sensing unit (left) is used for placing S parameter schematic diagrams before the micro-fluidic chip and after the micro-fluidic chip is placed;
fig. 6 is a schematic diagram showing the relationship between the reflection coefficients of the first sensing unit and the second sensing unit and the complex dielectric constant of the electrolyte solution injected into the microfluidic chip of the first sensing unit after the microfluidic chip is simultaneously placed in the first sensing unit and the second sensing unit according to the present invention: wherein (a) the reflection coefficient of the first sensing unit is in a relation with the complex dielectric constant of the electrolyte solution, and (b) the reflection coefficient of the second sensing unit is in a relation with the complex dielectric constant of the electrolyte solution.
The PCB board is provided with a PCB board; a resistance of 2.50 Ω; 3. a microstrip line I; 4. a microstrip line II; 5. a microstrip line III; 6. a microstrip line IV; 7. a microstrip line V; 8. a through hole; SMA connector; 10. a metal foil; a CSRR groove ring; 12. the region where the electric field intensity is maximum.
Detailed Description
The present invention will be described in further detail with reference to the following examples in conjunction with the accompanying drawings.
As shown in fig. 1, which is a schematic structural diagram of the present invention, the differential sensor of the present invention is composed of two identical sensing units, each of which includes a top microstrip line structure, a middle PCB board 1, and a CSRR groove ring 11 etched on a bottom metal sheet 10; the top microstrip line structure comprises five sections of microstrip lines: one end of the first microstrip line I is welded with one end of the microstrip line II 4 through a 50-ohm resistor 2, and the other end of the first microstrip line I is directly connected with the second microstrip line structure; one end of the fourth microstrip line IV6 is directly connected with one end of the third microstrip line iii 5, the other end is directly connected with one end of the fifth microstrip line V7, and the other end of the fifth microstrip line V7 extends out of a feed long pin for connecting with the SMA connector 9; the microstrip line II 4 and the microstrip line III 5 are coupled with the CSRR tank ring 11 on the bottom layer;
as shown in fig. 2, which is a schematic diagram of the distribution of electric field intensity of the present invention, each grooved metal CSRR structure is a square groove ring, the groove ring is provided with an opening, wherein the portion between two grooves connected at right angles opposite to the groove ring opening is a region 12 with maximum electric field intensity, which is sensitive to the change of complex dielectric constant of the electrolyte solution, so that a microfluidic chip is placed in the region for measuring the complex dielectric constant of the electrolyte solution;
the sensor design of the invention was carried out in a three-dimensional electromagnetic simulation software AnsysHFSS environment, with relevant dimensions obtained by the software, as shown in the following table:
| parameter(s)
|
l1 |
l2 |
l3 |
l4 |
l5 |
w
|
r
|
θ
|
| Numerical value (mm)
|
13.93
|
1.81
|
12
|
8.5
|
6
|
2.73
|
8.19
|
45o
|
| Parameter(s)
|
p1 |
p2 |
p3 |
p4 |
d
|
a
|
b
|
g
|
| Numerical value (mm)
|
1
|
0.5
|
2.5
|
3.5
|
2
|
19
|
10
|
0.5
|
| Parameter(s)
|
s
|
|
|
|
|
|
|
|
| Numerical value (mm)
|
1
|
|
|
|
|
|
|
|
The size of the middle layer PCB board is 78 multiplied by 50 multiplied by 0.767mm3High frequency board Rogers RO4350 (dielectric constant 3.66, permeability 1, dielectric loss 0.004, permeability loss 0)
Fig. 3 is a schematic structural diagram of a microfluidic chip according to the present invention, the chip is a U-shaped chip processed and manufactured from Polydimethylsiloxane (PDMS), and a microfluidic channel and a liquid inflow and outflow port are disposed in the chip. Before measuring the complex dielectric constant of the electrolyte solution, the electrolyte solution needs to be injected into the micro-flow channel from the liquid inflow port, and after the measurement is finished, the electrolyte solution needs to be discharged from the liquid outflow port;
fig. 4 is a schematic diagram of a three-dimensional hierarchical layout structure of the present invention, and a microfluidic chip is disposed on each sensing unit. The first microfluidic chip is used for measurement and can inject electrolyte solution inside, the second microfluidic chip is used for reference and does not inject electrolyte solution inside.
FIG. 5 is a schematic diagram of the S parameters of the sensor of the present invention: the reflection parameter change curves of the two sensing units are the same, the resonant frequency is 1.71GHz, and the Q value is 855. Therefore, the two sensing units do not interfere with each other, and the high measurement accuracy of the sensor is ensured by the high Q value.
Fig. 6 is a schematic diagram showing the relationship between the reflection coefficients of the first sensing unit and the second sensing unit and the complex dielectric constant of the electrolyte solution injected into the microfluidic chip of the first sensing unit after the microfluidic chip is placed in the first sensing unit and the second sensing unit of the present invention at the same time: the reflection coefficient of the first sensing unit and the complex dielectric constant of the electrolyte solution are in a relation schematic diagram, and the reflection coefficient of the second sensing unit and the complex dielectric constant of the electrolyte solution are in a relation schematic diagram. However, the dielectric constant of the electrolyte solution is increased from 1 to 80, the frequency offset of the sensor is 598MHz, the transverse sensitivity of the sensor is 7.47MHz, the transverse sensitivity of the sensor is far greater than that of the existing sensor based on the CSRR resonator, and the sensor has extremely strong practicability.
The invention has been described above with reference to the accompanying drawings, it is obvious that the invention is not limited to the specific implementation in the above-described manner, and it is within the scope of the invention to apply the inventive concept and solution to other applications without substantial modification, or with substantial modification.